Why can viewing an OWA attachment on a client machine be a security risk?

When viewing an OWA attachment on a client machine, a copy gets saved in the cache of that machine. If this computer is shared or a public terminal, (such as a kiosk, library etc) the attachment can be viewed, copied and saved by by unauthorized users.

There are third party add-ons, such as Messageware AttachView (www.messageware.com), which offer secure web access to OWA attachments by converting attachment files into secure web pages for over 300 file formats, providing secure access to attachments without the need of the application on the local computer.